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What is Specific Air Range?
- Distance travelled on a given quantity of fuel
- SAR=distance/fuel used
What is the equation for SAR?
What is SFC?
- Specific Fuel Consumption: relates the power output to the fuel being used at that specific time.
Show how SAR can be broken down into airframe and engine considerations
- therefore: SAR= TAS/Power x 1/SFC
- TAS/Power - airframe comsideration
- 1/SFC- engine consideration
In terms of airframe considerations explain why you would fly at Vimd to maximise range?
- TAS/Power = TAS/Drag x TAS = 1/Drag
- Therefore to maximise range you must minimise drag.
- Min drag occurs at Vimd
Why do we fly at RRS in practice as opposed to Vimd?
- Variation of the TAS/Power ratio at speeds near Vimd is negligible and a slightly higher speed will allow faster flight without undue loss of range.
- When flying at Vimd, any turb or manoeuvring will cause a loss of lift and height. To regain height, fuel is wasted therefore range will be reduced. A higher speed ensures a margin.
Explain the effect of weight on range
- Vimd is proportional to the root of AUW therefore Vimd will increase (therefore your TAS will increase)
- Increase in weight increases drag.
- As TAS and drag have increased, power required will increase therefore range will decrease.
Why does a low RPM increase range?
- Low RPM gives optimum SFC because:
- Friction loss is lower
- Ancillaries use less power
- Volumetric efficiency is improved
Do you want a high or low MAP to increase range and why?
- A greater weight of charge can be packed into each cylinder
Why does SFC improve as altitude is increased to FTH?
- Less throttling is required
- Less useful power output is wasted in exhausting spent mixture against ambient air pressure
- Colder air improves thermal efficiency
What happens if altitude is increased above FTH?
SFC deteriorates because the power required can only be achieved by increasing RPM.
How does temperature effect SFC?
- In cold air(denser), at a particular altitude RPM may be decreased to provide the same power because the power req'd is reduced and the power available is greater.
- SFC reduces because less energy is used and the thermal efficiency of the engine is improved.
Why does having the throttle fully open improve SFC?
No obstruction to gas flow therefore power output is improved
How do you configure for range in the airtrainer?
- Climb at full power to FTH for 23" (approx 7000')
- Set 110kts then reduce RPM to 1800
- Fuel flow to RR+5
- Prevent CHT exceeding 205 by increasing fuel flow
How do you minimise SFC?
- Operate at min RPM and with a weak mixture set
- Fly at FTH in cold air with the air filter removed
What two factors must be minimised to increase endurance?
- Ultimately require the least GFC in a given period:
- GFC=SFC x Power
- therefore must minimise SFC and Power
How does weight effect Vmp (and therefore endurance)?
An increase in weight increases drag due to the increased lift required. This increases Vmp and decreases endurance
How do you configure for endurance in the airtrainer?
- Fly as low as practical
- 1800RPM/18" (maintains 90-100kts)